Liquid membrane separation processes have been used for removal of dissolved substances such as ions from aqueous solutions, such as disclosed in U.S. Pat. No. 4,360,448(A). Said invention relates to a process for the removal of dissolved species from aqueous solutions, which comprises contacting said aqueous solution with an emulsion, said emulsion comprising an exterior phase which is characterized as being immiscible with said aqueous solution and yet permeable to said dissolved species, and an interior phase which contains a reactant, such as an ion exchange compound, capable of converting said dissolved species to a non-permeable form. The dissolved species permeate the exterior phase, into the interior phase where they are converted into non-permeable forms and thus retained in the interior phase of said emulsion. The aqueous solution, depleted in said dissolved species, is separated from said emulsion and the emulsion cycled for reuse. However, when multiple or unspecified ions or solutes are present in an aqueous solution or medium, such as a biological liquid it becomes increasingly complex to remove solutes by this or similar methods, since it would be necessary to device a specific reactant for each species to be removed. Another example of the use of a liquid membrane extraction process is described in (WO 87/002380) Production of Low-Ethanol Beverages by Membrane Extraction which relates to membrane extraction systems designed to selectively remove ethanol from wine and other beverages while retaining the water and numerous other organic constituents. Thus, liquid membrane separation methods have hitherto been developed for selective removal of solutes in, e.g. aqueous liquids. Seeing a need to selectively remove or extract water from aqueous liquid sources the present inventors have devised a liquid membrane process suitable to removal or extraction of pure water from an aqueous liquid using the selective water channel known from aquaporin proteins.
Fluorescent-based activity assays are well-established for soluble proteins, but not for membrane proteins. A likely reason for this is that membrane proteins are fragile when they are taken out of their natural environment—the biological membrane. Moreover, the accessibility to commercially available protein species has been restricted to only a few membrane proteins. This is related to the difficulty in expressing and purifying membrane proteins in large quantities (gram-scale). Membrane proteins typically retain their function upon reconstitution into a biomimetic membrane that sufficiently mimics the protein's natural environment. There is today an unmet need for an assay for screening lipid membrane components for their usefulness in the creation of a biomimetic membrane formulation that meets the membrane protein requirements, i.e. specific hydrophilic and hydrophobic regions or layers. At the same time, such an assay would provide useful information about the folding state of a membrane protein.